1. Field of the Invention
The present invention relates to a printing apparatus, printing system, and printhead temperature retaining control method. Particularly, the present invention relates to a printing apparatus which prints an image on a print medium by discharging ink using a printhead, a printing system including the printing apparatus, and a temperature retaining control method for the printhead.
2. Description of the Related Art
These days, performance demand is growing for printing apparatuses used as a printer, copying machine, and facsimile machine. The printing apparatus is required to print high-resolution images like a silver halide photograph, in addition to high-speed printing and full-color printing. To meet these demands, an inkjet printing apparatus can discharge small ink droplets at high frequency. The inkjet printing apparatus is superior to printing apparatuses using other printing methods in terms of high-speed printing and high-quality printing. Of inkjet printing apparatuses, a printing apparatus which employs a thermal inkjet printing method of discharging ink using bubbles generated by a heater (electrothermal transducer) can print high-resolution images because nozzles can be arrayed at high density.
The thermal inkjet printing method (to be simply referred to as an inkjet printing method hereinafter) has the following features.
According to the inkjet printing method, a heater is energized to generate thermal energy and generate bubbles in the ink. The growth of the generated bubbles is greatly influenced by the ambient ink temperature. At the interface between the bubbles and the ink, a process in which gas molecules in bubbles fly into the ink, and a process in which liquid molecules in the ink fly out of bubbles occur. The temperature of the ink near the bubbles influences the latter process. If the ink temperature is high, many molecules fly out of the ink into the bubbles, and the bubbles grow relatively large. To the contrary, if the ink temperature is low, a relatively small number of molecules fly out of the ink into the bubbles, and the bubble size is smaller than that at high ink temperature. The bubble size influences the volume of the ink (to be referred to as an ink discharge amount hereinafter) pushed out of the nozzle.
In the inkjet printing apparatus, the amount of ink discharge is strongly influenced by the ink temperature near the heater (to be referred to as an ink temperature hereinafter). The amount of ink discharge is large when the ink temperature is high, and small when it is low.
According to the inkjet printing method, the temperature near the heater during printing is higher than that at the start of printing.
This is because not all the thermal energy generated by the heater contributes to the bubble generation energy. Residual energy after subtracting, from the thermal energy, energy used to generate bubbles is stored as thermal energy in neighboring ink or a member such as a printhead substrate. Even the stored thermal energy is dissipated by thermal conduction or thermal radiation. However, the heater supplies thermal energy during printing, so the ink temperature continues to rise if the dissipation amount of thermal energy is smaller than its amount of supply. The temperature of ink which is not used to print and does not receive thermal energy from the heater continues to drop until it reaches an equilibrium state with the ambient temperature. In other words, a portion at which data is printed at high ink temperature, and a portion at which data is printed at a temperature as low as room temperature exist on a print medium depending on the heater driving count, that is, print data.
For this reason, the amount of ink discharge changes between a high-temperature printed portion and a low-temperature printed portion. When an image such as a photograph is printed, density unevenness may appear in the image printed on a print medium, degrading the print quality.
To prevent variations in the amount of ink discharge depending on the ink temperature, there has conventionally been known a temperature retaining control method of suppressing variations in the amount of ink discharge. According to this method, the printhead is heated to a given temperature before the start of printing, and adjusted to retain the temperature in the printhead during printing. For example, Japanese Patent Laid-Open No. 6-278291 proposes a method of presetting a temperature (reference temperature) at which the variation width of the amount of ink discharge can be decreased, and adjusting the printhead temperature by heating the printhead substrate to the reference temperature.
Japanese Patent Laid-Open No. 2004-160685 proposes a temperature retaining control method of heating a printhead substrate and changing, in accordance with the print mode, a temperature (reference temperature) serving as a reference upon adjusting the printhead temperature. More specifically, in a print mode in which printing is performed at high speed, the reference temperature is set relatively high to reduce the recovery operation and increase the throughput. In a print mode in which printing is performed at high image quality, the reference temperature is set relatively low to decrease the amount of ink discharge and print at high resolution.
Japanese Patent Laid-Open No. 5-31906 discloses an inkjet printing apparatus which prints while maintaining a printhead at a temperature higher than the ambient temperature to suppress variations in the amount of ink discharge over a wide temperature range by PWM control.
However, the maximum temperature which the printhead reaches during printing greatly changes depending on print conditions such as print data and the heater driving count. For example, the printhead temperature does not rise so high when printing a document or an image at low print density or when printing an image in a small print area. In this case, the maximum temperature which the printhead reaches is often stabilized at a temperature as low as room temperature. At this time, if the printhead temperature is adjusted to a reference temperature higher than this temperature, a large amount of thermal energy needs to be applied for the temperature adjustment, increasing power consumption. A relatively long heating time is necessary to heat the printhead substrate to the reference temperature. The higher the target reference temperature becomes, the longer the heating time also becomes. As a result, the throughput of the printing apparatus decreases.
Problems in the conventional arts will be listed in detail below.
Japanese Patent Laid-Open No. 6-278291 proposes a method of raising the reference temperature to increase the amount of ink discharge in order to fill the space between dots when printing at low resolution. Upon printing at low resolution, the heater driving count decreases, and the maximum temperature which the printhead reaches during printing drops. Nevertheless, to raise the reference temperature, a large amount of thermal energy must be applied. This reference does not explicitly disclose a method of changing the reference temperature in accordance with the degree of temperature rise of the printhead. When a document or image is so printed as to keep constant at low temperature the maximum temperature the printhead reaches during printing, as described above, a large amount of thermal energy is wastefully applied.
Japanese Patent Laid-Open. No. 2004-160685 proposes a method of retaining a high printhead temperature to improve head recovery in order to reduce the recovery operation and increase the throughput in the high-speed print mode. This reference also proposes a method of retaining a low printhead temperature to decrease the amount of ink discharge in order to print an image at high resolution in the high-quality print mode. However, when print data such as a text requiring a small number of heater driving counts is used even in high-speed printing, the maximum printhead temperature remains at low level. Thus, a large amount of thermal energy is applied for a long time in order to keep the printhead temperature high. When high-quality print data like a photographic image is used even in high-quality printing, the heater driving count is high and the maximum printhead temperature reaches high level. If the printhead is maintained at low temperature, the printhead temperature greatly varies, and the amount of ink discharge also greatly varies. Accordingly, an image suffering conspicuous ink density unevenness is output.
Japanese Patent Laid-Open No. 5-31906 proposes a method of retaining a printhead temperature higher than the ambient temperature. However, when a high-quality image like a photographic image is printed, the maximum temperature the printhead reaches during printing may exceed the retained printhead temperature, and the amount of ink discharge may vary. When print data such as text data requiring a small number of heater driving counts is printed, the maximum printhead temperature remains relatively low, but the printhead is retained at high temperature. This results in wastefully consuming power.
As summarized, when temperature retaining control is executed based on a reference temperature higher than the maximum temperature which the printhead actually reaches during printing, ink density unevenness upon variations in the amount of ink discharge can be reduced. However, a large amount of thermal energy is wastefully applied, increasing power consumption. If temperature retaining control is executed based on a reference temperature lower than the maximum temperature which the printhead actually reaches, the printhead temperature greatly varies, and ink density unevenness appears in an output image.